In the practical process of electroerosive drilling of the type described, it has generally been recognized to have been essential that the machining liquid be supplied to the machining gap from the tubular electrode under elevated pressure and at a regulated high velocity or rate of flow throughout the machining operation.
For example, it has been shown that such a thin hole can be electroerosively drilled at a removal rate as high as 10 to 30 mm/min with a highly slender metallic tubular electrode composed of, say, copper or brass and using as the machining liquid a water liquid having a conductivity of 150 to 250 .mu.A (or a specific resistance of 5.times.10.sup.2 to 10.sup.5 ohm-cm in general) when the liquid is projected from the tubular electrode under an elevated pressure of, say, 30 kg/cm.sup.2 and the machining current is applied to generate a succession of electrical discharges across the machining gap.
For high-speed electroerosive drilling, it has been recognized that the machining current density should be as high as 1000 amperes/cm.sup.2 and that the machining gap should be traversed by the machining liquid at a rate as high as 2 to 5 cc/ampere/minute in conjunction with the machining current employed. Such a high-flow rate of the machining liquid should be maintained to assure a prompt removal of machining products from the gap. It has also been recognized that removal of gap products may be promoted by applying an ultrasonic vibration to the flowing machining liquid or to the tool electrode or to both.
As electroerosive material removal proceeds, the tubular electrode needs to be advanced into the workpiece to progressively form the desired hole in the workpiece. To ensure high drilling accuracy, this advance should be effected with smoothness and thus at a constant or regulated rate but this has been recognized to be difficult to achieve with the conventional arrangement.
It has now been found that the difficulty arises mainly because the rate of flow of the machining liquid from the internal bore of the tubular electrode into the machining gap tends to fluctuate in an uncontrolled fashion, especially when it is delivered under an elevated pressure, say, in excess of 20 kg/cm.sup.2 to achieve a high drilling speed. Thus, the rate of flow tends to fluctuate due to variations of mechanical precision in machining and finishing the internal bore of the tubular electrode which is as thin as 1 mm diameter and to dimensional variations of the machining end portion of the thin tubular electrode prepared by cutting. Due to these variations, an irregular pressure drop is created in the internal bore of the tubular electrode and tends to cause a fluctuation in the flow rate of the machining liquid into the machining gap. Furthermore, the length of the tubular electrode tends to change due to the wear and erosion of its machining tip portion which may unavoidably occur during machining operations.
The loss of pressure .DELTA.P within the internal bore of a tubular electrode may, on the assumption that the liquid flow is laminar, be expressed as follows: ##EQU1## where .mu.: viscosity constant, Q: the rate of flow of the machining liquid by volume, d: the inner diameter of the tubular electrode, l: the length of the tubular electrode and v: the flow velocity. By substituting certain actual values for d and assuming that v is 1 m/sec, .DELTA.P and Q can be calculated as shown in Table 1 below.
TABLE 1 ______________________________________ d (mm) .DELTA.P (Kg/cm.sup.2) Q (cc/min) ______________________________________ 0.1 64 1.8 0.15 29 4.2 0.2 16 7.2 0.25 10 11.4 ______________________________________
It should be noted in this connection that for example, where the tubular electrode has an inner diameter of 0.15 mm (and an outer diameter of 0.3 mm) and the machining current ranges from 2 to 3 amperes, the water machining liquid must be delivered at a pressure as high as in excess of 40 kg/cm.sup.2 to establish a flow rate by volume of 7 to 10 cc/min.
In the conventional liquid delivery arrangement, a pump of delivery pressure of 50 to 100 kg/cm.sup.2 has been used and may be driven by a motor of 1 to 2 horsepower. Such a pump of more than enough delivery pressure is required to reduce pulsation to a minimum while allowing excess liquid to be released through a relief valve and the excess energy to be converted into heat. Even with such an arrangement, it has been extremely difficult to maintain the delivery rate of flow of the machining liquid into the machining gap at a desired level prior to or during a given machining operation due to a large pressure drop as referred to above across the thin tubular electrode and due to the aforesaid unavoidable dimensional variations thereof.